Which muscles to workout together?

For the past year I've been splitting my workout like this most of the time.

Day1
Chest, Shoulders, Triceps

Day2
Back, Biceps, Traps

Day3
Legs, Abs

Then repeat the three days, followed by a rest day. Then repeating the process over.

I would start with the big compound exercises first(bench press, deadlift, squats, etc), then move into isolation sets.

Before I use to do something like this, and I feel like I made gains quicker this way.
Day1
Chest, Back,Shoulders

Day2
Biceps,Triceps, Traps

Day3
Legs,Abs

I understand why its good to do the opposite muscles the same day(1 muscle group rests while you work the other), but I keep thinking its better to work certain muscles together like chest, tricep, and shoulder, because you hit them all doing any kind of press(incline, decline, military, etc..)

sorry i should have been more clear.
I meant I workout 6 days a week, and my muscles split into 3 groups/days. So I work each set of muscles twice a week. And then I have 1 day completely off. Hopefully that clears things up.

In my head, my schedule seems very logical, but I feel like I'm not getting the results I would expect.

Honestly bro, 3 days on and 1 off is the way to go.
You should put the leg day in the middle to give your arms a little extra recovery, despite the isolations.
day 1: Chest and Tris
day 2: Legs and abs
day 3: back and bis
day 4: Rest
Repeat.

Honestly bro, 3 days on and 1 off is the way to go.
You should put the leg day in the middle to give your arms a little extra recovery, despite the isolations.
day 1: Chest and Tris
day 2: Legs and abs
day 3: back and bis
day 4: Rest
Repeat.

Hey I am doing this exact split but where would I add cardio in there. I am trying to gain a bit but also trying to burn the fat that is over top of my abs. I use gold standard weigh as well.

Is it ideal to have Chest & tri, bi & back on the same days? I was doing it that way, but it seemed better to mix them up so I wasn't pre-exhausting my tri's while doing chest. I went to Chest & Back and bi & tri days, with a rest day between.. Any flaw in that?

At 157lb, you should still be doing a fullbody routine. 6 times per week is wasted volume/time. Your muscles would actually grow more from doing a full body split M/W/F, for example. You would do less volume, yes, but each body part would be worked 3x per week with REST in between. Your muscles aren't large enough to require that kind of volume.

Is it ideal to have Chest & tri, bi & back on the same days? I was doing it that way, but it seemed better to mix them up so I wasn't pre-exhausting my tri's while doing chest. I went to Chest & Back and bi & tri days, with a rest day between.. Any flaw in that?

If you don't mind not working your chest out to full capability, then do Tri's on the same day. Or visa versa. Full chest day is working great for me. Bench press (incline, decline, flat) & dumbbell flys (incline, decline, flat). 45 minutes (hour at-the-most). If i had time i'd also do bi's on the same day! Good luck bud!